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Old 09-19-2011, 10:46 PM   #1 (permalink)
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Sprocket changes

I see a lot posted about sprocket changes (like theycallmebryan's 250r) and I was wondering if someone could explain how they work.

Switching the back sprocket out for a lower number of teeth can increase the fuel mileage by lowering rpms? At what point do the benefits cease? theycallmebryan has a 39 on the back, but you can buy 35's...

A small guide on this mod would be great!

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Old 10-10-2011, 12:58 AM   #2 (permalink)
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hey i was just lurking about and saw no one has responded to you. what a shame. i have seen some post using all types of technical talk and nothin here . well i am no smart guy but i think (and i'll bet some one will correct me) it can be explained simply, like if you look at the circumfrence (tooth count is used) of your gears. think about two gears meshing (not much difference for this example) a little one driving a big one, little one might do several rpm before big one does even one rpm. changing the ratio (tooth count)of one (or both)gear(s) can give you more max speed but strain the motor or it can lower the maxspeed and let you climb a wall. that probably confused you more but hey i tried on my bike stock gears i think are 15/45, 15 being the drive front gear and 45 being the rear. i changed the front to a 16 so with that you could sort of say i gained 1 more chain link per revolution meaning the rear wheel is moving 1 link faster (sorta of) making the bike go faster at the same rpm. or you could look at it as i can slow down rpm's to maintain same speed as before. i am looking at lowering the tooth count in the rear which is like raising the tooth count in the front. ok i give up now by the way tire size has to be looked at too to understand more in depth the "big picture"
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Old 10-10-2011, 07:27 AM   #3 (permalink)
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When I did the Vetter Rally in 1984, I went to an industrial supply and got the largest front and smallest rear I could fix on my bike. I would not recommend this for the long term because those industrial sprockets are not as long lasting, but it worked for a single event.

If your bike is still pretty close to stock, you can go down several teeth on the rear or up one on the front and you will notice a difference. If you gear up too much, the bike will be harder to ride.

Back when I was racing motocross, I had a 2-stroke YZ400 with good midrange torque. I hopped up the engine and added one tooth to the front, and it was killer. I didn't save much gas, but it got big holeshots.
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Old 10-28-2011, 02:50 AM   #4 (permalink)
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Ive been playing with my fazer 600 2001 sprocketts and went from 15/48 to 17/45 now trying 17/42 - sitting behind a lorry commuting all motorway at around 56 im averaging around 76 mpg which at 140.0p per litre saves me around two pound a day sat at 70 mph. im hoping this new sprockett will give me more.

the only snag is sitting at that speed but what the hell im saving money and when i do get to work my lorry has been limited to 52 mph so im kind of used to slowww.

check this out - Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator

very helpful site
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Old 10-28-2011, 08:04 AM   #5 (permalink)
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Lowering engine RPM and increasing load by changing final drive ratios will get the engine in a better range of the BSFC map. Most bikes are geared for two riding which requires higher ratios for decent acceleration. Eliminate the second rider and lowering the final drive ratios works well to increase mileage.

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Old 10-28-2011, 10:17 AM   #6 (permalink)
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Increasing the sprocket tooth count and therefore diameter of the front or drive sprocket makes the wheel spin faster in relation to engine speed. Decreasing the sprocket tooth count of the rear or driven sprocket makes the wheel spin faster in relation to engine speed. And vice versa. Its all about leverage.

On my 200 I went from a 15 front and 46 rear to a 15 front and 40 rear. It's really tall for this little engine. Engine speed at 60 mph dropped from a calculated 6950 rpm to 6000. Search for 'sprocket calculator' and get your manual out to plug in values for your particular bike to estimate changes you would like to make.

One thing to consider is that when gearing 'up' or 'taller' like this, first gear isn't affected as much as the top gear. The change is evenly spread out through all gears, spacing the rpm change between each gear out a little more. Most people that ride on the street like the taller and wider spaced gearing over the typical close ratio, short gearing that comes stock. On a racetrack or when you are accellerating at maximum repeatedly, the close ratio gears are useful for keeping the engine near maximum output rpms through successive gear changes. Most street riding doesn't require this.
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Old 10-28-2011, 10:49 AM   #7 (permalink)
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Quote:
Originally Posted by autotravis View Post
I see a lot posted about sprocket changes (like theycallmebryan's 250r) and I was wondering if someone could explain how they work.
Take the rear sprocket's number of teeth, divide by the front sprocket's teeth.
That's the final drive ratio.
[let's say you have a 15 front, 45 rear setup, drive ratio is 3 - 1 revolution of the wheel takes 3 revolutions of the output shaft of the engine]

The higher that ratio is, the more rpm the engine will need to do to turn the rear wheel for a given speed.
The lower the ratio is, the less rpm needed for a given speed.

Lower rpm means less fuel is going to the engine over a same distance.
That's what your car's overdrive or top gear does.

Say you could do 60mph @ 6000 rpm using the common standard ratio of 3 (45:15)
An extreme 34:17 setup would give a ratio of 2
3:2 = 1.5
6000:1.5 = 4000 rpm @ 60 with the 34:17 setup

A more moderate 36:16 gives 2.25
3 : 2.25 = 1,33 and 4500 rpm @ 60

Quote:
Switching the back sprocket out for a lower number of teeth can increase the fuel mileage by lowering rpms?
Yes.
You can also add teeth to the front.

1 more tooth at the front equals about 3 teeth on the rear
(as most bikes are set up around 1:3)

When adding teeth to the front sprocket, make sure the chain still clears the chain cover, and any possible pushrod to the clutch in front of it
(Suzuki aircooled 4-in-lines like the bandit come to mind).

Quote:
At what point do the benefits cease?
Well, the engine must be able to - literally - pull it off.
Something with plenty of torque at low rpm would do well.

And the rider must be OK with the reduced acceleration - others who are less concerned about mileage would likely call it sluggish


As Old Mech already pointed out, the lower rpms also load up the engine more during acceleration as you make it work harder at low rpm, hence it produces those HP more efficiently.
That's a second benefit.

It also reduces vibrations, most of the time, though you should avoid bringing your most frequently used engine speed into the rpm range where the engine's natural vibrations are - on a 4 in line, this is often around 4000 rpm.

See where your bike passes off more vibrations than usual to you, and avoid getting in that rpm range by regearing it.
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Old 10-30-2011, 06:08 PM   #8 (permalink)
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just an update - stuck the 42 on the back and took it to work (94miles), sat at 3.5k revs around 90kmph used 5.21 litres which is around 82 mpg. so well happy, didn't feel sluggish either
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Old 10-30-2011, 07:03 PM   #9 (permalink)
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On a dyno an old tech GM 2.5 liter 4 cylinder would use 1 unit of fuel to produce 20 HP. At 50 HP the fuel consumption was 1.5 units of fuel.

Basically that means the 20 HP per unit went to 33.3 HP per unit when the load was increased. In a motorcycle with a small engine you can increase the load easily with a significant sprocket change and lower final drive ratio.

In many cases in bikes the first gear is very low to get the bike going with a max load, which in many cases is as much as the bike's weight itself, unlike a car where it is a much lower percentage of total vehicle weight.

Decreasing the total aerodynamic drag reduces the load, but it can be offset to a point by lower BSFC. The combination of lower final drive ratio, say from 3 to 1 to 2 to one, will increase the BSFC and offset the loss (in BSFC due to lower load) to improved aerodynamics. Coast down testing with some aero improvements in a bike and a good tuck position of the rider can really make a significant difference in highway speed MPG.

I once read that 90% of the energy required to get a bicycle to 30 MPH is aero drag. This is why some bike (bicycle not motorcycle) riders have hit 70 MPH when they were closely following a vehicle with a fairing on the rear of the lead vehicle to provide maximum reduction in aero drag to the bicycle.

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Old 10-31-2011, 09:16 AM   #10 (permalink)
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All of that being said, you will usually see 100 to 150 rpm change per tooth change. (one front tooth equalling 3 would be 300 to 450 rpm) REBELGEARS.com has been my first choice on custom sprockets, they will make any rear sprocket with any tooth and pitch size, fast and reasonable too.

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